I've found for plastic BudJones chips that have changed color the only treatment necessary is to rub some oil on them. It doesn't take much... rub you finger aside your nose and then rub the chip. It will usually change color before your very eyes. I had some of the common Sahara red/white coin inlay chips that looked almost brown. With a little oil, they looked bright red as did all the others.

I wouldn't use sandpaper on these smooth plastic chips. It's ok on the old clay chips that have a rough surface anyway. Should't be necessary anyway as the plastic is not porous like the clay and any surface contaminants will not penetrate.